Human Blood - a Circulatory Medium
Blood is the main circulatory medium in the human body. It's a fluid connective tissue. Blood has the following main functions:
- It carries oxygen and nutrients to different body cells and takes away carbon dioxide and other waste products from the cells.
- It helps in regulating body temperature.
- Fights infection, repairs damaged tissues.
- Helps in growth and development of the body
Components of blood
The main components of blood are plasma and formed elements. These are being discussed below:
Plasma: Plasma is an extracellular fluid but the largest component of the blood, a clear yellowish liquid portion of the blood in which cellular components i.e. RBCs, WBCs, platelets, etc. remain suspended. Besides, plasma contains water, salts, enzymes, antibodies, and other proteins.
Formed elements: These elements remain enclosed in a plasma membrane. They have a definite structure and shape. The Formed elements are Red blood corpuscles (Red blood cells or RBCs also called erythrocytes), White blood corpuscles (White blood cells or WBCs also called Leukocytes) and Platelets (also called Thrombocytes). These elements are being discussed here:
Erythrocytes or RBCs: They are known for their bright red color, contain a red color pigment called hemoglobin—an iron-containing protein molecule. They carry oxygen from the lungs to the tissues and organs of the body and return carbon dioxide received from different body organs and tissues to the lungs for purification of the blood. RBCs account for about 45 % of the blood volume. Production of RBCs is controlled by a hormone called erythropoietin which is primarily produced by the kidneys. Production of RBCs takes place in the bone marrow—a spongy tissue inside bones—where the cells reside till maturation. Then being matured they are released into the bloodstream. RBCs have no nucleus, they can easily change shape. The average lifespan of an RBC is about 120 days.
Leukocytes or WBCs: They help the body in fighting infections. They account for less than 1% of total blood volume, yet they play a key role in defending the body against diseases and infections. All WBCs have nuclei. A neutrophil is the most common type of WBC which gives immediate response against infection. It survives for less than a day. Bone marrow constantly produces neutrophils. A lymphocyte is another major WBC, it has three main groups: T lymphocytes and B lymphocytes and natural killer. T lymphocytes, commonly known as T cells, produced from stem cells in the bone marrow but develop in the thymus gland. They play a key role in the immune response. B lymphocytes, commonly known as B cells originate and develop in bone marrow, responsible for making antibodies. Antibodies are proteins that fight against foreign substances called antigens in the body. The third main WBCs i.e Natural killer or NK cells are part of the innate immune system. They quickly respond to foreign substances in the body. NK cells are specialized cells in killing cancerous and virus-infected cells.
Thrombocytes or Platelets: Platelets are the smallest fragments among all blood cells. They control excessive bleeding and are responsible for blood clotting. They do not have nuclei. In their non-active form, they look like a plate. When there is an injury or a wound in a body part or a blood vessel gets damaged a signal is sent out to platelets. Platelets reach the site of injury or bleeding, grow and spread into sticky tentacles with the help of fibrin. Fibrin is an insoluble and tough protein that is produced at the site of injury. Platelets contain fibrinogen, a soluble protein produced by the lever, that gets converted into fibrin by the action of thrombin, a clotting enzyme. Fibrin molecules entangle platelets building up a spongy mass around the wound thereby making a clot to stop the further loss of blood.
Platelets circulate in the bloodstream after formation and remain alive for 8 to 10 days.
Healthy platelet count:
- Normal platelet count ranges from 1.5 lakhs to 4.5 lakhs per micro-liter of the blood.
- If the platelet count falls below 150000 per microliter of circulating blood then a condition occurs called thrombocytopenia which might result in easy bruising, frequent bleeding from the gums, nose, or gastrointestinal tract.
- If the platelet count rises above 450000 per microliter of circulating blood then a condition occurs called thrombocytosis. This condition may result in spontaneous blood clots in arms and legs which may even increase the chances of heart attack and stroke.
Main facts on blood:
- The body of an average human being contains about 5 liters of blood.
- RBCs, WBCs, and platelets are formed in bonemarrow before entering the bloodstream.
- Blood transports oxygen and nutrients around the body and removes cellular waste, besides performing a wide range of other vital functions.
Blood groups: are categorized based on the antibodies and antigens present in the cell. Antibodies are protein found in blood plasma while antigens are the proteins molecule found on the surface of RBCs. Mainly four blood groups have been determined. Besides, there is another protein that can be present (+) or absent (-) in a blood group called the Rh factor. So, all in all, there are a total of 8 blood groups: (A+, A-, B+, B-, O+, O-, AB+, AB- ).
- Rh- blood is given to Rh- patients only while Rh+ or Rh- may be given to Rh+ patients.
Blood Donors and Recipient compatibility
- Before blood transfusion, the blood group of the recipient must be correctly matched with that of the donor. An incompatible blood donation can endanger a patient's life.
- Some common blood-related disorders are hemophilia, anemia, blood clotting, and cancer.
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This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.
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